Manufacturing processes using chemicals, in either gaseous or liquid form, often require that the chemical flow be metered. Metering of a liquid chemical is complicated, since temperature must be controlled. This often involves controlled heating of the pipe sections downstream from the source to prevent freezing or boiling or to prevent condensation of an evaporated liquid. Similarly, pipe sections carrying gaseous chemicals which have been heated at the source must be heated to maintain the gas temperature.
The heating of the pipe sections is in itself not difficult. One relatively easy method of heating is to treat the pipe as a heating element through which an electrical heating current is passed. Stainless steel, a preferred metal in the construction of pipe sections for handling gaseous chemicals, has electrical properties similar to those found in electrical heating elements.
A major problem associated with the heating of the pipe sections by the application of a current is that the section must be electrically isolated. Nonconductive pipe elements, such as glass or ceramics, may not be used to electrically isolate the heated pipe because of the danger of breakage. Non-metallic connecting tubes and hoses have several disadvantages including lower mechanical strength, thermal limitations, and the risk of introducing contaminants. Additionally, a proper junction between metals and nonmetals is hard to achieve because of the introduction of dead space, virtual leak, and other undesirable compromises.
A gas line isolator, described in U.S. Pat. No. 4,602,809 to Ross et al. electrically isolates two conductive tubing sections using complementary coupling members which are rigidly attached to the tubing and separated by a dielectric spacer. U.S. Pat. No. 4,519,634 to Hand illustrates a pipe fitting which couples a natural gas pipe line to a building pipe line while insulating the metallic casing of the gas line from any electric current passing through the building line. U.S. Pat. Nos. 3,574,357 and 4,284,297 are exemplary of pipe fittings incorporating thermal insulation.
The pipe fittings of Ross and Hand couple together and electrically isolate adjoining pipe sections. However, rigidly attaching the coupling members to the pipe sections creates various problems when used on pipelines which transport metered chemicals. A pipe fitting which avoids these problems is highly desirable.
Since the coupling members are rigidly attached to the pipe, any torsional stress introduced during coupling will affect the pipe sections by reducing the working life of the components. This also makes the assembly and maintenance of the pipe fitting more difficult. Using coupling members which may be moved along the pipe section would reduce the stresses introduced to the pipe section, and would facilitate installation and maintenance of the pipe fitting.
Rigidly connecting the coupling members to the pipe sections by brazing risks the introduction of contaminants from flux residues to the chemicals being carried by the pipe line. The flux residues may also collect at the juncture between the pipe sections, permitting leakage of chemicals from the pipe line. Movable coupling members, which are not mounted to the pipe section, would avoid this problem.
When the coupling members are rigidly attached to the pipe sections, some axial displacement is necessary to separate the pipe sections. In the tightly spaced, complex pipe lines used to transport chemicals, being able to separate the pipe sections by radial displacement is of particular advantage. Movable coupling members would facilitate radial displacement.
The insulating material used to provide electrical isolation between the pipe sections will wear before the metallic components and need to be replaced. When the coupling members are rigidly attached to the pipe sections, this may involve completely disassembling the pipe fitting. Thus, using an insulating component which may be replaced without removing the pipe fitting would facilitate maintenance and increase the working life of the pipe fitting.